Synovial effusions of the knee will be systematically studied in patients with rheumatic diseases. Special attention will be paid to the diagnosis, the volume of the effusion, its hydrostatic and colloid osmotic pressures, its content of glucose, of respiratory gases, and of selected serum proteins and the rate of clearance from the joint of both free and albumin bound radioiodine. From these variables, we expect to draw inferences as to the effective synovial blood flow, the synovial lymph flow, the permeability of the synovium to proteins, the mean hydrostatic pressure within the synovial microvasculature, and the inter-relationships of these parameters. We expect, for instance, that the hydrostatic pressure of effusions will correlate negatively with pH, pO2, glucose concentration, and effective synovial blood flow while correlating positively with pCO2 lactate concentration, and synovial permeability to proteins. At present, the hydrostatic pressure of effusions is rarely measured. It may be, however, that this simple parameter is one of the most important variables that clinicians can assess in the diagnosis and management of patients with swollen joints. These studies also make it possible to ask questions about the course and prognosis of synovial effusions. Do our physiological parameters predict synovial responsiveness to corticosteroid injections? What changes do corticosteroids induce in the synovium or its microvasculature? Are oral anti-inflammatory agents effective in changing effective blood flow, lymphatic drainage, permeability to proteins, or other parameters under study? Are there physiological patterns which are diagnostically useful in evaluating individual arthritic patients? What are the observable changes induced by myxedema, hypoalbuminemia, and other edematous states? What effects do heat and exercise have on the synovium? In these and other questions, we will use joint effusions as a window through which we may evaluate the synovium and its alterations by disease.